The present invention relates in general to vehicle speed control systems and in particular to an improved input circuit for an electronic vehicle speed control unit.
Vehicle speed control units are widely known in the art and are adapted to maintain a vehicle at a constant predetermined speed despite varying engine loads, such as are imposed by the course of the road or the wind. Because of their reliability, accuracy, and inexpensiveness, fluid-actuated vehicle speed control units have become the dominant factor in the marketplace. Such speed control units typically utilize vacuum from the vehicle engine manifold as the actuating force. The vacuum is selectively supplied to a bellows connected to the engine throttle linkage, thereby increasing or decreasing the throttle position to advance or retard the vehicle speed. The control unit for such a system typically compares a signal representing the actual vehicle speed with a signal representing the desired vehicle speed and adjusts the vacuum level supplied to the bellows accordingly.
Frequently, electronic control circuits are utilized to control the supply of vacuum to the bellows. However, such electronic control circuits are subject to unreliable operation or failure because of the large amount of electrical noise present in the vehicle engine. Such noise is typically generated by the vehicle ignition system and can adversely affect the signal representing the actual speed of the vehicle. Accordingly, it would be desirable to provide an input circuit for the electronic vehicle speed control unit which is resistant to such spurious noise signals.